UK Low Insulation Resistance Test

[DazzaL]

Hi guys,

Hoping someone can clarify some things here. I had a socket replaced in my kitchen recently and as part of this, the electricians did an Insulation Resistance test and told me the reading was very low at just over 1 mega ohms. They advised me to basically rewire the ring (at significant cost) as it needed to be 300MOhms plus from the consumer unit (actually replace the unit too as they wanted to split the ring into 2 or 3 rings as it turned out the ring covered the kitchen and lounge plus a bedroom. All in all it would cost in the region of £4-5k to rectify this once re-tiling/plastering was done.

I spoke to another electrician friend of mine about this and the first thing he asked was: did they disconnect all appliances from every socket before testing? Well, no they didn't do that - there were loads of things plugged in still (tv, smart speakers, wifi routers and lots of kitchen appliances all plugged in and switched in the on position). At this he just said forget it, the test is invalid unless you disconnect everything first.

So is it fair to say that this may be a non-issue (the low reading of 1Mohms) as they didn't disconnect everything first? or should I get it retested with everything out (I was quoted £400 by the same company to do more diagnostics though they discouraged that as an option and wanted to go straight into a full on rewiring job).

 

[Dartlec]

It depends exactly how they did the test, but it should involve sending 500V DC down the cables, which modern electrical equipment does not always like, so yes everything should have been disconnected or at least isolated (turning off at a socket does not always isolate fully).

Without further information it is hard to say whether there really is an issue but there are several red flags in how you've described it.

A genuine reading of 1M should lead to further investigation but not necessarily immediately to a rewire unless the cable is rubber or very old.

What was the further diagnostic work they quoted for ? An eicr which checks the wiring installation properly, should find any issues and I would expect to cost a fair bit less than £400 unless you have a very large house or are in Central London maybe.

How old is the wiring? Has it ever been tested before?

 

[DazzaL]

It depends exactly how they did the test, but it should involve sending 500V DC down the cables, which modern electrical equipment does not always like, so yes everything should have been disconnected or at least isolated (turning off at a socket does not always isolate fully).

Without further information it is hard to say whether there really is an issue but there are several red flags in how you've described it.

A genuine reading of 1M should lead to further investigation but not necessarily immediately to a rewire unless the cable is rubber or very old.

What was the further diagnostic work they quoted for ? An eicr which checks the wiring installation properly, should find any issues and I would expect to cost a fair bit less than £400 unless you have a very large house or are in Central London maybe.

How old is the wiring? Has it ever been tested before?

the 400 was to test every socket and wiring between them (15 sockets) on that one circuit (3 rooms in total). no not in central London - just in the suburbs. as to the age I'm not sure. i've only been in the property 10 years and this is an extension so not that old I guess. its not been tested since I've been here.

 

[Dartlec]

Testing just that circuit should not be taking long enough to get anywhere near £400 to be honest. It should take no more then 1 hour to get proper readings on the entire circuit and have a very good idea of if there is a problem and where it may be (for example a leg of cable between two sockets) .

You may be best looking for another electrician to do that, or perhaps a full eicr if you want a better idea of the full installation.

There may well be some electricians on here that would be willing to help.

If you are on the Eastern end towards Kent then it's within my area.

 

[Wilko]

Hi - It is entirely possible the low IR reading they have obtained can be resolved without rebuilding your kitchen etc. As per @Dartlec, I would be happy to help you with a second opinion if you were nearby.

 

[pc1966]

PVC wire, typically used from the 60s onwards, is normally very long lived unless it is overheated or physically damaged. The old rubber wire is another sorry story altogether!

Assuming the 1M is still there with everything disconnected, then it is the sort of resistance you get from either a charred wire, damp/dirty socket, or one touching something like plasterboard. All are serious enough to investigate ASAP but don't need a full rewire.

Occasionally we see on this forum examples of bad/counterfeit cable that shows this sort of degradation but it is less likely if the property has been good for 10+ years.

As above, I suspect a good electrician will find the fault at one socket point, junction box, or limited section of damaged cable for a lot less!

 

[DazzaL]

Hi - It is entirely possible the low IR reading they have obtained can be resolved without rebuilding your kitchen etc. As per @Dartlec, I would be happy to help you with a second opinion if you were nearby.

Thanks for the feedback. I'm around the Harrow area so will look to get someone in my area to give it another look with everything disconnected.

Yeah I didn't want to go down the rebuild path as it seemed excessive for now.

 

[Vortigern]

Last time I got a low IR reading took half a day to find and remedy. Cost; £150. As to 300 Mohms, not so, 2 Mohms and above meets BS7671. Although saying that I would ideally want to get that up a bit and may check to see if anything can be done to improve it.

 

[Lucien Nunes]

As above, results are invalid unless all appliances disconnected, and no reason to expect 300MΩ. It's the sort of reading one gets with all new, clean, fittings on a warm dry day and one can get attached to the idea of always wanting to see values this high. But even a little condensation on a damp day can bring it down; the pass level is a hundred times lower for a reason.

 

[DefyG]

was very low at just over 1 mega ohms.

BS7671 Table 64 actually quotes Minimum values of insulation resistance to be 1Mohm! upto 500v

So if 'just over 1Mohm' has been recorded recorded it is 'compliant' but worth getting a second opinion and checked out if still low after disconnecting all appliances.
Probably only half a days work at most to determine cause, not £400 worth!

 

[andyampere]

It used to be only 0.5M Ohms. And in reality circuits with even lower readings can work perfectly fine.
Let's put this in it's place, you are having your tail pulled mate. If you are worried about it call in a qualified electrician, preferably one that is a member of NAPIT or the NICEIC for peace of mind. You can always try 'Which' they have a good reliable database of professionals.

 

[pc1966]

I always wondered about that 1M threshold, to me it seems way too low for any normal setup.

Yes, if you have a warehouse of old dusty and damp connectors all putting in 10-20M or so in parallel, but otherwise down at a few meg at a localised source always makes think of a nail through a cable in to plasterboard or similar.

 

[DefyG]

call in a qualified electrician, preferably one that is a member of NAPIT or the NICEIC

We dont know that they weren't and they possibly are, if working for a reputable kitchen company!
I know plenty of 'approved' contractors that would still even sc....w their gran over for a quick buck!!!!

 

[freddo]

I always wondered about that 1M threshold, to me it seems way too low for any normal setup.

Don't forget 1Mohm Is 1,000,000 ohms! Works out at something like 0.05Watts at 230V, so not going to be starting any fires from excessive heating, even if the 1Mohm was localised to one small position.

 

[FatAlan]

Was any other testing carried out on the circuit other than IR?

 

[pc1966]

Don't forget 1Mohm Is 1,000,000 ohms! Works out at something like 0.05Watts at 230V, so not going to be starting any fires from excessive heating, even if the 1Mohm was localised to one small position.

It is not the 1M that concerns me as such, it is why it is there and what might happen with a bit of extra damp, movement, or another rodent having a bite.

 

[cliffed]

Anything less than 2M ohms need investigation,& thus should sort out that 1 M ohm reading.
I doubt that a rewire is needed,just a decent caring Sparkie

 

[pc1966]

Anything less than 2M ohms need investigation,& thus should sort out that 1 M ohm reading.
I doubt that a rewire is needed,just a decent caring Sparkie

Definitely.

The problem with simple pass/fail thresholds like, say, 1M is it fails to take in to account the extent of what is being tested. So 1M on a circuit of 100-200m cable and dozens of sockets or light fittings in dusty and damp conditions is not so worrying, where as a simple 10m radial to an appliance in a dray flat means something serious has gone wrong.

 

[Risteard]

BS7671 Table 64 actually quotes Minimum values of insulation resistance to be 1Mohm! upto 500v

So if 'just over 1Mohm' has been recorded recorded it is 'compliant' but worth getting a second opinion and checked out if still low after disconnecting all appliances.
Probably only half a days work at most to determine cause, not £400 worth!

It should be remembered that this is for the entire installation though, and not just for a tiny section of cable. So you would either need to conduct the test on the installation in its entirety or calculate the resistances in parallel.

 

[UNG]

If you are worried about it call in a qualified electrician, preferably one that is a member of NAPIT or the NICEIC for peace of mind. You can always try 'Which' they have a good reliable database of professionals.

Yes get a properly qualified electrician in that knows what they are doing. Using scheme "electricians" doesn't necessarily give any guarantee of their ability other than being able to pay the annual fees